Chapter 14 Ocean Intraplate Volcanism (from Plumes)

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Chapter 14 Ocean Intraplate Volcanism (from
Plumes)
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Ocean islands and seamounts Commonly associated
with hot spots

Figure 14-1. After Crough (1983) Ann. Rev. Earth
Planet. Sci., 11, 165-193.
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Ocean islands and seamounts Commonly associated
with hot spots
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Types of OIB Magmas

• Two principal magma series
• Tholeiitic series (dominant type)
• Parental ocean island tholeiitic basalt, or OIT
• Similar to MORB, but some distinct chemical and
  mineralogical differences
• Alkaline series (subordinate)
• Parental ocean island alkaline basalt, or OIA
• Two principal alkaline sub-series
• silica undersaturated
• slightly silica oversaturated (less common
  series)

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Tholeiitic and Alkaline examples

• Modern volcanic activity of some islands is
  dominantly tholeiitic (for example Hawaii and
  Réunion).
• Other islands are more alkaline in character (for
  example Tahiti in the Pacific and a concentration
  of islands in the Atlantic, including the Canary
  Islands, the Azores, Ascension, Tristan da Cunha,
  and Gough)

Hawaii data, both tholeiitic and alkaline
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Hawaiian Scenario

• Cyclic pattern to the eruptive history
• 1. Pre-shield-building stage somewhat alkaline
  and variable
• 2. Shield-building stage begins with tremendous
  outpourings of tholeiitic basalts
• This stage produces 98-99 of the total lava
  in Hawaii

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Hawaiian Scenario

• 3. Waning activity more alkaline, episodic, and
  violent (Mauna Kea, Hualalai, and Kohala). Lavas
  are also more diverse, with a larger proportion
  of differentiated liquids
• 4. A long period of dormancy, followed by a late,
  post-erosional stage. Characterized by highly
  alkaline and silica-undersaturated magmas,
  including alkali basalts, nephelinites, melilite
  basalts, and basanites
• The two late alkaline stages represent 1-2 of
  the total lava output

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Evolution in the Series

• Tholeiitic, alkaline, and highly alkaline

Figure 14-2. After Wilson (1989) Igneous
Petrogenesis. Kluwer.
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• Alkalinity is highly variable
• Alkalis are incompatible elements, unaffected by
  less than 50 shallow fractional crystallization,
  this again argues for distinct mantle sources or
  generating mechanisms

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Trace Elements
High Field Strength Elements (HFS or HFSE)
elements (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are
also incompatible, and are enriched in OIBs gt
MORBs Ratios of these elements are also used to
distinguish mantle sources. For example The
Zr/Nb ratio N-MORBs are generally quite high
(gt30) OIBs are low (lt10)
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Trace Elements

• The large ion lithophile (LIL) trace elements (K,
  Rb, Cs, Ba, Pb2 and Sr) are incompatible and are
  all enriched in OIB magmas with respect to MORBs
• The ratios of incompatible elements have been
  employed to distinguish between source reservoirs
  
• N-MORB the K/Ba ratio is high (usually gt 100)
• E-MORB the K/Ba ratio is in the mid 30s
• OITs range from 25-40, and OIAs in the upper 20s
• Thus all appear to have distinctive sources

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Trace Elements REEs
Note that ocean island tholeiites (OITs
represented by the Kilauea and Mauna Loa samples)
overlap with MORB and are not unlike E-MORB The
alkaline basalts have steeper slopes and greater
LREE enrichment than the OITs. Some fall within
the upper MORB field, but most are distinct
Figure 14-2. After Wilson (1989) Igneous
Petrogenesis. .
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Isotope Geochemistry

• Isotopes do not fractionate during partial
  melting of fractional melting processes, so will
  reflect the characteristics of the source
• OIBs, which sample a great expanse of oceanic
  mantle in places where crustal contamination is
  minimal, provide incomparable evidence as to the
  nature of the mantle

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Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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Mantle Reservoirs

• 1. DM (Depleted Mantle) N-MORB source

Figure 14.8. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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2. BSE (Bulk Silicate Earth) or the Primary
Uniform Reservoir
Figure 14.8. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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• 3. EMI enriched mantle type I has lower
  87Sr/86Sr (near primordial)
• 4. EMII enriched mantle type II has higher
  87Sr/86Sr
• (gt 0.720), well above any reasonable
  mantle sources

Figure 14.8. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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5. PREMA (PREvalent MAntle)
Figure 14.8. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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Simple Mixing Models
Ternary All analyses fall within triangle
determined by three reservoirs

• Binary
• All analyses fall between two reservoirs as
  magmas mix

Figure 14.7. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
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Note that all of the Nd-Sr data can be reconciled
with mixing of three reservoirs DM EMI and
EMII since the data are confined to a triangle
with apices corresponding to these three
components. So, what is the nature of EMI and
EMII, and why is there yet a 6th reservoir (HIMU)
that seems little different than the mantle
array?
Figure 14-6. After Zindler and Hart (1986),
Staudigel et al. (1984), Hamelin et al. (1986)
and Wilson (1989).
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• Pb is quite scarce in the mantle
• Low-Pb mantle-derived melts susceptible to Pb
  contamination
• Incompatibles U, Pb, and Th are concentrated in
  continental crust
• 204Pb is non-radiogenic.
• 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb
  increase as U and Th decay
• Oceanic crust also has elevated U and Th content
  (compared to the mantle)
• So are sediments derived from oceanic and
  continental crust
• So Pb is a sensitive measure of crustal
  (including sediment) components contaminating
  mantle isotopic systems
• 93.7 of natural U is 238U, so 206Pb/204Pb will
  be most sensitive to a crustal-enriched component

9-20 238U ? 234U ? 206Pb 9-21 235U ?
207Pb 9-22 232Th ? 208Pb
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Figure 14-7. After Wilson (1989) Igneous
Petrogenesis. Kluwer.
207Pb/204Pb vs. 206Pb/204Pb data for Atlantic and
Pacific ocean basalts Geochron simultaneous
evolution of 206Pb and 207Pb in a
rock/reservoir line on which all modern
single-stage (not disturbed or reset) Pb isotopic
systems, such as BSE (Bulk Silicate Earth),
should plot. Notice NONE of the oceanic
volcanics fall on the geochron. Nor do they fall
within the EMI-EMII-DM triangle, as they appear
to do in the Nd-Sr systems. The remaining mantle
reservoir HIMU (high mu) proposed to account for
this great radiogenic Pb enrichment pattern
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• m 238U/204Pb (evaluate uranium enrichment)
• HIMU reservoir very high 206Pb/204Pb ratio
• Source with high U,
• yet not enriched in Rb (has modest 87Sr/86Sr)
• Old enough (gt 1 Ga) to observed isotopic ratios
  
• HIMU model
• Subducted and recycled oceanic crust ( seawater)

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• EMI and EMII
• High 87Sr/86Sr require initially high Rb long
  time to 87Sr
• Correlates with continental crust (or sediments
  derived from it)
• Oceanic crust and sediment are other likely
  candidates

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HIMU is also 208Pb enriched, so this reservoir is
enriched in Th as well as U
Dupré and Allègre (1983),
Figure 14.10 After Wilson (1989) Igneous
Petrogenesis. Kluwer. Data from Hamelin and
Allègre (1985), Hart (1984), Vidal et al. (1984).
207Pb/204Pb data (especially from the N
hemisphere) linear mixing line between DM and
HIMU, a line called the Northern Hemisphere
Reference Line (NHRL) Data from the southern
hemisphere (particularly Indian Ocean) departs
from this line, and appears to include a larger
EM component (probably EMII)
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He Isotopes
Other isotopic systems that contribute to our
understanding of mantle reservoirs and dynamics

• Noble gases are inert and volatile
• 4He is an alpha particle, produced principally by
  a-decay of U and Th, enriching primordial 4He
• 3He is largely primordial (constant)
• The mantle is continually degassing and He lost
  (cannot recycle back)
• 4He enrichment expressed as R (3He/4He)
• R unusual among isotope expressions in that
  radiogenic is the denominator
• Common reference is RA (air) 1.39 x 10-6

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He Isotopes

• N-MORB is fairly uniform at 81 RA suggesting an
  extensive depleted (degassed) DM-type N-MORB
  source

Figure 14.12  3He/4He isotope ratios in ocean
island basalts and their relation to He
concentration. Concentrations of 3He are in cm3
at 1 atm and 298K.After Sarda and Graham (1990)
and Farley and Neroda (1998).
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He Isotopes
OIB 3He/4He values extend to both higher and
lower values than N-MORBs, but are typically
higher (low 4He). Simplest explanations
Figure 14.12  3He/4He isotope ratios in ocean
island basalts and their relation to He
concentration. Concentrations of 3He are in cm3
at 1 atm and 298K.After Sarda and Graham (1990)
and Farley and Neroda (1998).
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He Isotopes
PHEM (primitive helium mantle) is a He3/He4
mantle end-member reservoir with near-primitive
Sr-Nd-Pb characteristics. PHEM no longer exists
due to radiogenic increases in 4He.
Figure 14.13  3He/4He vs. a. 87Sr/86Sr and b.
206Pb/204Pb for several OIB localities and MORB.
The spread in the diagrams are most simply
explained by mixing between four mantle
components DM, EMII, HIMU, and PHEM. After
Farley et al. (1992).
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He Isotopes Summary
Shallow mantle MORB source is relatively
homogeneous and depleted in He OIBs have more
primordial (high) 3He/4He, but still degassed and
less than primordial (100-200RA) values,
consistent with our deeper mantle ideas. Again,
PHEM concept may be like that more primitive
mantle reservoir, prior to natural increase in
radiogenic He and contamination. Current Lower
than PHEM 3He/4He in OIBs may be due to
recycled crustal U and Th
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Re/Os system and Os Isotopes
187Re ? 187Os Both are platinum group elements
(PGEs) PGEs ? core or sulfides depending on
whether or not they are compatible. Os is
compatible during mantle partial melting (goes
into ? solids as a trace in sulfides, so they
dont leave the mantle), but Re is moderately
incompatible (goes into ? melts and, eventually,
crust silicates) The mantle is thus enriched in
Os relative to crustal rocks and crustal rocks.
Crustal rocks have higher Re and lower Os and
develop a high (187Os/188Os) as Re decays, which
should show up if crustal rocks are recycled back
into the mantle. Re is Rhenium and Os is
Osmium
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Os Isotopes plus the FOZO
All of the basalt provinces are enriched in 187Os
due to high Re decaying, over the values in
mantle peridotites and require more than one
187Os-enriched reservoir to explain the
distribution.
187Re ? 187Os
Figure 14.13  187Os/188Os vs. 206Pb/204Pb for
mantle peridotites and several oceanic basalt
provinces. Os values for the various mantle
isotopic reservoirs are estimates. After Hauri
(2002) and van Keken et al. (2002b).
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Other Mantle Reservoirs
FOZO (focal zone) another convergence
reservoir toward which many trends approach. Thus
perhaps a common mixing end-member
Figure 14.15. After Hart et al., 1992).
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• EMI, EMII, and HIMU too enriched for any known
  mantle process...must correspond to crustal rocks
  and/or sediments
• EMI
• Slightly enriched
• Deeper continental crust or oceanic crust
• EMII
• More enriched
• Specially in 87Sr (Rb parent) and Pb (U/Th
  parents)
• Upper continental crust or ocean-island crust
• If the EM and HIMU continental crust (or older
  oceanic crust and sediments), only deeper
  mantle by subduction and recycling
• To remain isotopically distinct could not have
  rehomogenized or re-equilibrated with rest of
  mantle

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The Nature of the Mantle

• N-MORBs involve shallow melting of passively
  rising upper mantle
• ? a significant volume of depleted upper
  mantle (DM which has lost lithophile elements to
  melts which ended in late fractionation rocks,
  and which has lost He).
• OIBs seem to originate from deeper levels.
• Major- and trace-element data ? the deep
  source of OIB magmas (both tholeiitic and
  alkaline) is distinct from that of N-MORB.
• Trace element and isotopic data reinforce this
  notion and further indicate that the deeper
  mantle is relatively heterogeneous and complex,
  consisting of several domains of contrasting
  composition and origin. In addition to the
  depleted MORB mantle, there are at least four
  enriched components, including one or more
  containing recycled crustal and/or sedimentary
  material reintroduced into the mantle by
  subduction, and at least one (FOZO or PHEM) that
  retains much of its primordial noble gases.
• MORBs are not as homogenous as originally
  thought, and exhibit most of the compositional
  variability of OIBs, although the variation is
  expressed in far more subordinate proportions.
  This implies that the shallow depleted mantle
  also contains some enriched components.

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Mantle Questions

• Is the mantle layered (shallow depleted and
  deeper non-depleted and even enriched)?
• Or are the enriched components stirred into the
  entire mantle (like fudge ripple ice cream)?
• How effective is the 660-km transition at
  impeding convective stirring? This depends on the
  Clapeyron slope of the phase transformation at
  the boundary!

Pick any two points on an equilibrium curve dDG
0 DVdP - DSdT
Clapeyron Eq.
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No Effect Retards Penetration Enhances
Penetration
? 2-Layer Mantle Model
? Whole-Mantle mixing
Figure 14.16. Effectiveness of the 660-km
transition in preventing penetration of a
subducting slab or a rising plume
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Case 1 dP/dT at 660 km is negative

• Figure 1.14. Schematic diagram of a 2-layer
  dynamic mantle model
• The 660 km transition is a sufficient density
  barrier to separate lower mantle convection
• Only significant things that can penetrate this
  barrier are vigorous rising hotspot plumes and
  subducted lithosphere
• Subducted lithosphere sinks to become
  incorporated in the D" layer where they may be
  heated by the core and return as plumes). After
  Silver et al. (1988).

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Case 2 dP/dT at 660 km is positive
Figure14.17. Whole-mantle convection model with
geochemical heterogeneity preserved as blobs of
fertile mantle in a host of depleted mantle.
Higher density of the blobs results in their
concentration in the lower mantle where they may
be tapped by deep-seated plumes, probably rising
from a discontinuous D" layer of dense dregs at
the base of the mantle. After Davies (1984) .
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2-layer Model for Oceanic Magmatism
Continental Reservoirs
DM
OIB
EM and HIMU from crustal sources (subducted OC
CC seds)
Figure 14-10. Nomenclature from Zindler and Hart
(1986). After Wilson (1989) and Rollinson (1993).
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Whole Mantle Model for Oceanic Magmatism
Figure 14.19. Schematic model for oceanic
volcanism. Nomenclature from Zindler and Hart
(1986) and Hart and Zindler (1989).